In association with the recent tendency for a higher degree of integration in semiconductor devices with the thickness of 0.25 micron rule or below and having multilayered wiring, as a space between the metal wires becomes narrower, impedance between the metal wires increases due to electrostatic induction, and therefore there is a strong concern about delay in a response speed or increase in power consumption. To overcome this problem, it is necessary to make as low as possible a dielectric constant of an inter-layer insulating film provided between a semiconductor substrate and a metal wiring layer such as an aluminum wiring layer or between metal wiring layers, and/or an inter-metal insulating film provided between metal wirings.
These insulating films provided for the purpose as described above are generally formed on a semiconductor substrate by the vapor-phase growth method such as the CVD method (Chemical Vapor Deposition Method) or the coating method such as the spin coat method.
In the case of the insulating film prepared by the latest technology in the CVD method (Refer to, for instance, the patent document 1), although an insulating film with the specific dielectric constant of 3 or below can be obtained, it is generally said that it is difficult to prepare an insulating film with the specific dielectric constant of around 2.5 or below, and like in the conventional coating method, there is also a defect that the film strength of the coating film becomes lower as the specific dielectric constant becomes lower. Further in a case of a CVD insulating film made from polyaryl resin, fluorine-added polyimide resin and fluorine resin, or in a case of an insulating film formed with the coating liquid, the specific dielectric constant can be lowered to around 2, but the adhesiveness to a surface of a substrate is not good, and further also the adhesiveness to the resist material used for fine mechanical processing and the like is low, and further the chemical resistance and the resistance against oxygen plasma are disadvantageously low.
Further with a coating film obtained by using a coating liquid for forming a silica-based coating film containing hydrolysates or partial hydrolysates of alkoxysilane and/or halogenated silane, although a coating film with the specific dielectric constant of 3 or below can be obtained, but it is difficult to achieve the specific dielectric constant of 2.5 or below, and the adhesiveness with a coated surface is rather low, which is disadvantageous.
The present inventors made strenuous efforts for solving the problems described above, and found out that, by using a) a coating liquid for forming a silica-based coating film with a low dielectric constant including a reaction product between alkoxysilane and/or halogenated silane or a hydrolysate thereof and silica fine particles (Refer to, for instance, patent document 2), b) a coating liquid for forming a silica-based coating film with a low dielectric constant including alkoxysilane and/or halogenated silane or a hydrolysate thereof and easily-decomposable resin which decomposes or vaporizes at a temperature of 500° C. or below (Refer to, for instance, patent document 3), c) a coating liquid for forming a silica-based coating film with a low dielectric constant including polysiloxane as a reaction product between alkoxysilane and/or halogenated silane or a hydrolysate thereof and silica fine particles and easily decomposable resin which decomposes or vaporizes at a temperature of 500° C. or below (Refer to, for instance, patent document 4), or d) coating liquid for forming a silica-based coating film With a low dielectric constant including alkoxysilane and/or halogenated silane or a hydrolysate thereof and an organic template material (Refer to, for instance, patent document 5), it is possible to form a coating film with the dielectric constant of 3 or below and excellent adhesiveness to a coated surface, high film strength, and chemical resistance such as alkaliproof characteristics, excellent cracking resistance and capable of ensuring smoothness of a coated surface, and further having high resistance against oxygen plasma and excellent process adaptability such as etching workability.
However, the present inventors repeatedly carried out experiments for forming a silica-based coating film with a low dielectric constant on various types of semiconductor substrates using a coating liquids and known methods for forming a coating film (spin coat method or other coating methods), and found out that, although a coating film having the characteristics as described above can be obtained, when it is tried to form a coating film with the specific dielectric constant of 2.5 or below, the film strength drops and a coating film having the Young's modulus of 6.0 GPa (giga pascal) or more strongly desired from the semiconductor industries can hardly be obtained in the stable state.
On the other hand, researchers of the US California University have proposed a method of forming a zeolite coating film (silica zeolite coating film having the MFI crystalline structure) on a semiconductor substrate using a suspension prepared by separating and removing particles having relative large diameters respectively from the zeolite fine particles obtained by hydrolyzing tetraethyl orthosilicate (TEOS) dissolved in ethyl alcohol in the presence of tetrapropyl ammonium hydroxide (TPAOH). The zeolite coating film obtained by this method has the Young' modulus of 16 to 18 GPa, but the moisture-absorption characteristics is high, so that the zeolite coating film absorbs moisture (water vapor) contained in the air and rapidly increases the specific dielectric constant thereof (for instance, from 2.3 to 3.9), and in that case the practicability of the zeolite coating film is disadvantageously lost. To overcome this problem, there have been proposed some methods including a method of keeping the specific dielectric constant of this coating film in the range from 2.1 to 2.3 by subjecting the zeolite coating film obtained as described above to sylilation to make the surface hydrophobic (Refer to, for instance, non-patent document 1, and patent document 6).
In order to carry out the sylilation (processing by the CVD method) as described above, however, in addition to the need of capital investment, complicated operations are required, so that the cost becomes substantially high. Further as the size of zeolite particles included in the coating film is large around 20 nm, a surface of the obtained zeolite coating film is substantially rough, and for instance, polishing is required for smoothing the surface. Further when the zeolite coating film is subjected to the processing for make it hydrophobic, only the surface is made hydrophobic, and therefore when a wiring pattern, a through hole or the like is formed by subjecting the coating film to fine mechanical processing such as resist application or etching, portions not having been converted to the hydrophobic state are exposed with moisture absorbed from the portions, and as a result the specific dielectric constant of the coating film is worsened (namely made higher), which is disadvantageous.
The present inventors made concentrated efforts for solving the problems as described above, and found out that the problems can be solved by preparing a coating liquid for forming a coating film having novel compositions and properties as described below, applied the coating liquid on a substrate, subjecting the substrate to heating step and curing step in succession for forming an amorphous silica-based coating film and completed the present invention.    [Patent document 1] Japanese Patent Laid-Open Publication No. 2000-349083    [Patent document 2] Japanese Patent Laid-Open No. 1997-315812    [Patent document 3] International Application Publication WO 00/18847    [Patent document 4] International Application Publication WO 00/12640    [Patent document 5] Japanese Patent Laid-Open Publication No. 2002-30249    [Patent document 6] U.S. patent application Publication US 2000/0060364 A1    [Non-patent document ] Advanced Material 2001, 13, No. 19, October 2, Page 1453-1466